Method to fabricate thin insulating film

ABSTRACT

The invention grows SiO 2  films over silicon at temperatures as low as room temperature and at pressures as high as 1 atmosphere. The lower temperature oxidation is made possible by creation of oxygen atoms and radicals by adding noble gas(es) along with oxidizing gas(es) and applying RF power to create plasma. The invention also fabricates silicon nitride films by flowing nitrogen containing gas(es) with noble gas(es) and applying RF power to create plasma at pressures as high as one atmosphere. In addition, the above processes can also be performed using microwave power instead of RF power to create plasma.

BACKGROUND OF THE INVENTION

1. Field of Invention

The present invention relates to a method of fabricating insulatingfilms for application with thin film transistors (TFTs) and metalinsulator oxide (MOS) transistors.

2. Description of Related Art

Forming an insulating film, such as SiO₂, is a significant step in themanufacture of transistors, such as silicon MOSFET (Metal OxideSemiconductor Field Effect Transistor). Formation of SiO₂ film onsilicon is performed at temperatures that are usually higher than 1000°C. and in the presence of chemical species that oxidize the silicon.This process is known as thermal oxidation. The thermal oxidationprocess has undesirable side-effects, such as redistribution of thedopant profiles in the semiconductors, since significant diffusion ofdopants occurs at the high temperatures that are used in this process.

Thin film transistor (TFT) devices, which have a basic structure that issimilar to that of a typical MOSFET, have been used for displayapplications, such as liquid crystal displays (LCD) and organicelectroluminescence displays (OELD). Such devices require an SiO₂ layerto be formed at a temperature that is below 430° C., since thesedisplays use an optically transparent substrate, such as glass, whichcannot withstand higher temperatures. For such TFT applications,currently deposited Sio₂ films are used which are of inferior quality,and which also form an inferior interface with silicon compared to SiO₂that is produced by oxidation of silicon; thereby adversely affectingthe TFT performance. Thus, it is required that the oxidation process beperformed at temperatures that are as low as possible.

Recently, T. Ueno et al. JJAP 39, pp. L327 (2000) (reference 1) and T.Ohmi et al. Pro. of IDW (1999), pp. 159 (reference 2) reported thatapplying microwave field to a gas mixture of noble gas and oxygengenerates plasma containing atomic oxygen and oxygen radicals, whicheasily oxidize silicon to form an SiO₂ film even at temperatures thatare lower than 500° C.

The low temperature processes mentioned in the above references involvegeneration of plasma. Generally, plasma processes are performed atpressures of 1 torr (133 Pa) or less. Additionally, the process-chamberneeds to be evacuated to an even lower pressure (base pressure) beforethe process gases are introduced into the chamber. Thus, these plasmaprocesses require the use of expensive vacuum tools. The maintenance ofthe vacuum tools further adds to the cost. Vacuum tools also occupyexpensive clean room space. Additionally, the microwave plasma processesused above are also limited in their applications. The microwaveprocesses are suitable for semiconductor processing where the maximumsize for the substrate is 300 mm in diameter. For the case of TFTs, thesubstrate size is much larger and approaches 1000 mm×1000 mm in thelatest generation equipment. For such large substrate, the microwaveplasma process is not suitable.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an inexpensive methodfor fabricating high quality insulating films at low temperatures.Further, it is an object of this invention that such insulating films befabricated over large substrate, such as those used in TFT fabrication.

In accordance with the invention, high quality SiO₂ films are fabricatedby plasma excitation of noble gases along with oxygen at the pressuressubstantially close to atmospheric-pressure (about 100 kPa). Thisprocess completely eliminates the need of using vacuum tools, making theequipment and the process very inexpensive compared to equipment andprocesses used for making similar insulating films in TFT andsemiconductor industries. Additionally, radio frequency (RF) power inthe MHz range can be used in this process, which makes it possible toapply the process to large substrate.

Additionally, the same process can be used to form silicon nitride byusing the nitriding species (such as NH₃, N₂, etc.) along with noblegases, and creating plasma at pressures that are substantially close toatmospheric pressure.

The method of creating RF plasma at pressures that are substantiallyclose to atmospheric-pressure is advantageous from cost and simplicityperspectives. As a matter of course, even if the process pressure isreduced to be as low as 1 kPa, the processes can be performed withinexpensive vacuum tools.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic of a plasma oxidation process using RF power inaccordance with the invention;

FIG. 2 is a C-V curve for the MOS capacitor fabricated using the SiO₂film that is grown using the disclosed method of example 1 in accordancewith the invention; and

FIG. 3 is a C-V curve for the MOS capacitor fabricated using the SiO₂film that is grown using the disclosed method of example 2 in accordancewith the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In accordance with the invention, a noble gas and a reagent gas (such asan oxidizing agent) were introduced into a chamber. The chamber containstwo electrodes (see FIG. 1) between which RF power was applied. Thepressure in the chamber was substantially close to one atmosphere (about100 kPa), and no vacuum equipment was employed in this process. In theroutine plasma processing currently being employed in the industry,which is done at pressures in the order of 100 Pa or lower, theformation of plasma is relatively easy. In these routine plasmaprocesses, typically the RF power density is in the order of severalhundred milli-watts/cm², and the spacing between the electrodes is inthe order of 20 mm. However, it becomes increasingly difficult to formplasma as the pressure is increased. It has also been almost impossibleto create plasma at pressures that are substantially close to oneatmospheric using only the reagent gases, such as oxygen. Adding a noblegas, such as helium or argon, in large proportion to the reagent gas,makes it easier to form plasma at higher pressures. However, formingplasma at pressures that are as high as atmospheric pressure is stillquite difficult. In order to create plasma at the pressures that aresubstantially close to one atmospheric pressure, RF power density andthe electrode spacing had to be changed significantly, and a noble gashad to be added to a reagent gas. The inventor found that the RF powerdensity of several watts/cm² in the MHz range was necessary. A plasmawas more effectively formed when the distance between the electrodes, onwhich RF power was applied, was less than 5 mm.

In addition to oxidation of silicon, the above process can also beapplied to nitridation of silicon at pressures that are substantiallyclose to one atmosphere. The nitridation can be carried out by in thepresence of nitrogen containing-compounds (such as N₂, NH₃ etc.) insteadof oxygen containing compounds along with noble gas(s) and sustainingplasma. These nitride films can be subjected to further processing, suchas annealing (thermal annealing, rapid thermal annealing, laserannealing), to further enhance the quality of the films.

It should also be noted that these processes were performed at thepressure of 1 atmosphere (close to 100 kPa). If needed, it is possibleto reduce pressure down to 1 kPa with inexpensive vacuum tools. Thus,the above processes can be performed in a gas pressure range of 1 kPa to110 kPa.

Instead of RF, microwaves can be used to create plasma at relativelyhigher gas pressures (for example, 1 kPa or higher), although microwaveshave been used to create plasma at pressures that are lower than 1 kPa.

EXAMPLE 1

According to the above process guidelines, the oxidation of silicon wasperformed in the following manner. Helium gas and oxygen gases wereintroduced into a chamber in which silicon substrate to be oxidized wasplaced. The percent of oxygen in the gas mixture was 2%. The pressurewas one atmosphere (about 100 kPa). The plasma was sustained by RF powerat a frequency of 40 MHz. The RF power density was 3W/cm². Thetemperature was 200° C. The electrode spacing was 1.5 mm. The siliconsubstrate, 0.5 mm thick, was placed on the lower electrode. FIG. 2 showsa high frequency C-V curve of a metal oxide semiconductor (MOS)capacitor fabricated after growing the Sio₂ films on p-type siliconusing the above conditions. The C-V curve is close to the ideal case andhas no hysteresis. The defect state density at the Si—SiO₂ interface(interface state density or D_(it)) near the silicon mid-gap energy wasdetermined using low-frequency C-V characterization. The D_(it) valuewas found to be close to 5×10¹⁰ cm⁻²-eV⁻¹. Such a low value of D_(it)indicates an excellent, device-quality interface.

In order to determine how this D_(it) value compares with SiO₂ filmsformed in a similar way using routine low-pressure plasma, oxidationexperiments were performed at 133 Pa (1 torr) pressure. The otherexperimental conditions were as follows. RF power density=500milli-watts/cm², electrode spacing=20 mm, temperature=200 degree C., andO₂/He=2%. The D_(it) value obtained for the Si—SiO₂ interface for thislow-pressure plasma oxidation process was 6×10¹¹ cm⁻²eV⁻¹. Thus, thedefect density at the Si—SiO₂ interface for the SiO₂ films fabricatedusing the plasma process at pressures that are substantially close to 1atmosphere is much lower than that for the SiO₂ fabricated at the lowerplasma pressure of 133 Pa.

For comparing the interface state density of the Si—SiO₂ interfacefabricated by the disclosed method to that of a thermally grown SiO₂film, similar silicon wafers were oxidized at 1000° C. The densityinterface state in the thermal oxidation case was found close to 2×10¹¹cm⁻²eV⁻¹. Thus, the defect density at the Si—SiO₂ interface for the SiO₂films fabricated at 200° C. using the plasma process at pressuresubstantially close to 1 atmosphere is also lower than that for the SiO₂fabricated at 1000° C. using thermal oxidation, indicating high qualityof interface obtained using the disclosed process.

EXAMPLE 2

In the disclosed process of example 1, the silicon substrate, that is toreact with plasma, was placed between the electrodes, between which theplasma was created. It is possible to create plasma by applying RF powerin a separate chamber, and then transferring the plasma to anotherchamber in which a silicon substrate, that is to react with the plasma,is placed. Such a process is referred to as a remote plasma process, andis expected to reduce plasma damage to the substrate, and to furtherenhance the quality of the SiO₂ films. MOS capacitors were fabricatedusing the SiO₂ films produced by remote plasma oxidation. For the SiO₂formation, the pressure was 1 atmosphere (about 100 kPa), thetemperature was 200° C., the O₂/He gas flow ratio was 1.5%, and thepower density was 70 W/cm². FIG. 3 shows the C-V characteristics of theMOS capacitor. In spite of using very high power density, no hysteresisis shown in FIG. 3, showing low damage to SiO₂ films by remote plasmaprocess.

In the above two examples, oxygen is used as an oxidizing agent. Howeverother oxidizing agents, such as N₂O or H₂O or a mixture of variousoxidizing agents, can also be used along with the noble gas to performthe oxidation process.

These SiO₂ films can be subjected to annealing, such as thermalannealing or laser annealing, to further enhance their properties,preferably in an ambient that contains hydrogen, such as forming gasambient.

It was also observed that adding a small amount of fluorine-containingcompound (such as HF, NF₃, CF₄) to the process gas mixture enhanced theoxidation rate compared to that observed without the addition offluorine containing compound. Adding chlorine-containing compound alsoexhibited similar enhancement of the oxidation rate. The addition offluorine or chlorine compound also enhanced the film quality.

The disclosed process can be further refined by adding nitrogen ornitrogen containing compound along with oxidizing agent(s) and noblegas(es) to enhance the reliability of a device incorporating the silicondioxide films.

EXAMPLE 3

The SiO₂ film produced by the disclosed process was incorporated in thefabrication process of TFTs. Two kinds of TFTs were fabricated, i.e., 1)Reference TFTs, which were fabricated by a routine TFT process, and 2)Plasma oxidation TFTs, which were fabricated by the following steps, ofwhich step 3 includes one of the methods related to the presentinvention.

1. On a glass substrate, an amorphous silicon layer, with a thickness of50 nm, was deposited by an LPCVD method.

2. The amorphous silicon films were laser annealed by a XeCl pulsedlaser to change it into polycrystalline silicon with an approximategrain size of 0.3 micrometers, and the polycrystalline silicon layer wassubsequently patterned by photolithography to make islands.

3. An SiO₂ layer (SiO₂ layer 1) with a thickness of 4 nm was fabricatedover the polycrystalline silicon applying RF power to oxygen-helium gasmixture at a pressure that was substantially close to 1 atmosphere(about 100 kPa). An SiO₂ layer (SiO₂ layer 2) was further deposited byECR CVD method so that the total thickness of a layer including SiO₂layer 1 and SiO₂ layer 2 was 120 nm.

4. Gate metal was deposited and patterned.

5. Source and drain regions were created by ion doping.

6. An isolation SiO₂ layer was deposited and patterned to formsource-drain contact holes.

7. Source-drain contact metal was deposited and patterned.

The reference TFTs for purposes of comparison were fabricated by anidentical process mentioned above except for fabricating SiO₂ layer instep 3. The reference TFTs did not contain the said SiO₂ layer 1.Instead, the entire SiO₂ layer was deposited by ECR CVD having athickness of 120 nm. Thus, the reference TFT process is similar to aroutine TFT process, whereas the plasma oxidation TFT process includesan additional SiO₂ layer (SiO₂ layer 1) in order to enhance theinterface between polycrystalline silicon and the SiO₂ layer.

When the performance of the two kinds of TFTs was evaluated andcompared, the plasma oxidation TFTs performed better than the referenceTFTs in all respects. An n-channel mobility value of 170 cm²V⁻¹sec⁻¹ forthe plasma-oxidation TFTs was obtained, compared to a value of 120cm²V⁻¹sec⁻¹ for the reference TFTs. A sub-threshold-slope value of 0.4V/decade for the plasma-oxidation TFTs was obtained, compared to a valueof 0.64 V/decade for the reference TFTs. An order of magnitude loweroff-current value for plasma oxide TFTs was obtained, compared to thereference TFTs. All of these results indicate a significantly betterperformance of the plasma oxidation TFTs.

What is claimed is:
 1. A method for fabricating SiO₂ film, comprising:providing a silicon-containing substrate in a chamber; introducing oneor more gases, which contain at least one oxidizing gas, into thechamber, a pressure in the chamber being controlled in a range of from 1kPa to 110 kPa; applying RF power in a MHz range, the RF power beingapplied under such a condition that: a plasma containing at leastreactive oxidizing species is generated, and the reactive oxidizingspecies reacts with the silicon of the substrate to convert at least apart of the silicon into SiO₂.
 2. The method of claim 1, the applyingstep including applying RF power having a frequency in a range of 1 MHzto 100 MHz.
 3. The method of claim 1, the applying step includingapplying RF power between electrodes that are spaced apart by 5 mm orless.
 4. The method of claim 1, wherein the one or more gases furtherinclude one or more noble gases of helium, argon, neon, krypton, xenonor any one of a mixture of at least two chosen from the group consistingof helium, argon, neon, krypton and xenon.
 5. The method of claim 1,wherein the at least one oxidizing gas is oxygen, Ozone, H₂O, N₂O, orany one of a mixture of at least two chosen from the group consisting ofoxygen, Ozone, H₂O and N₂O.
 6. The method of claim 1, the applying stepincludes applying RF power at a pressure such that the ratio of partialpressure of said at least one oxidizing gas to the total gas pressureranges from 0.05 to 15 percent.
 7. The method of claim 1, furtherincluding the step of placing said silicon to be oxidized betweenelectrodes to which RF power is applied to create plasma.
 8. The methodof claim 1, the applying step including applying RF power such that anRF power density is in a range of 0.5 to 10 W/cm2.
 9. The method ofclaim 1, further including the steps of preventing said silicon to beoxidized from being placed between electrodes between which the plasmais created; and subsequently transporting the plasma created between theelectrodes by application of RF power to the silicon.
 10. The method ofclaim 9, the applying step including RF power such that an RF powerdensity is in a range of 1 to 100 W/cm2.
 11. The method of claim 1,wherein a temperature of the silicon during the reaction with thereactive oxidizing species is in a range of 20° C. to 700° C.
 12. Themethod of claim 1, wherein a temperature of the silicon-containingsubstrate during the reaction with the reactive oxidizing species is ina range of 20° C. to 430° C.
 13. The method of claim 1, furtherincluding the step of annealing that includes at least one of thermalannealing, rapid thermal annealing, and laser annealing of the SiO₂film.
 14. The method of claim 13, the annealing step including annealingin an ambient containing hydrogen.
 15. The method of claim 1, theapplying and reacting steps being performed in the presence offluorine-containing compound, the partial pressure of thefluorine-containing gas being less than 1 percent of the total gaspressure.
 16. The method of claim 1, the applying and reacting stepsbeing performed in the presence of chlorine-containing compound, thepartial pressure of the chlorine-containing gas being less than 5percent of the total gas pressure.
 17. The method of claim 1, theapplying step including applying RF power in the presence of nitrogen ornitrogen containing compound in addition to the at least one oxidizinggas.
 18. A method of fabricating silicon nitride film, comprising:providing a silicon-containing substrate in a chamber; introducing oneor more gases, which contain at least one nitriding gas, into thechamber, a pressure in the chamber being controlled in a range of from 1kPa to 110 kPa; applying RF power in a MHz range, the RF power beingapplied under such a condition that: a plasma containing at leastreactive nitriding species is generated, and the reactive nitridingspecies reacts with the silicon of the substrate to convert at least apart of the silicon into silicon nitride.
 19. The method of claim 18,wherein the one or more gases further include one or more noble gases ofhelium, argon, neon, krypton, xenon, or any one of a mixture of at leasttwo chosen from the group consisting of helium, argon, neon, krypton andxenon.
 20. The method of claim 18, wherein the at least one nitridinggas is nitrogen, NH₃, N₂O, or any one of a mixture of at least twochosen from the group consisting of nitrogen, NH₃ and N₂O.
 21. Themethod of claim 18, further including placing said silicon betweenelectrodes to which RF power is applied to create plasma.
 22. The methodof claim 18, further including the steps of preventing said silicon frombeing placed between electrodes between which the plasma is created, andsubsequently transporting the plasma created between the electrodes byapplication of RF power to the silicon.
 23. The method of claim 18,further including annealing that includes at least one of thermalannealing, rapid thermal annealing, and laser annealing of the siliconnitride film.
 24. The method of claim 23, the annealing step includingannealing in an ambient containing hydrogen.
 25. A method of fabricatingSiO₂ film, comprising: applying RF power in the presence of noblegas(es) and oxidizing gas(es) at a total pressure in a range of 1 kPa to110 kPa to create a plasma containing reactive oxidizing species; andsubsequently reacting the reactive oxidizing species with a silicon partof a substrate to convert at least a part of silicon into SiO₂.
 26. Amethod of fabricating silicon nitride film, comprising: applying RFpower in the presence of noble gas(es) and nitriding gas(es) at a totalpressure in a range of 1 kPa to 110 kPa to create a plasma containingreactive nitriding species; and subsequently reacting the reactivenitriding species with a silicon part of a substrate to convert at leasta part of silicon into silicon nitride.
 27. A method of fabricating aninsulator film, comprising: applying microwave power in the presence ofnoble gas(es) and reagent gas(es) at a total pressure in a range of 1kPa to 110 kPa to create a plasma containing reactive reagent species;and subsequently reacting the reactive species with a silicon part of asubstrate to convert at least a part of silicon into an insulator film.